1. Field of the Invention
The present invention relates to laser elements, and more particularly to laser elements of distributed feedback and distributed Bragg reflector types wherein a first periodic structure for optical feedback is provided along an optical waveguide path.
2. Description of the Related Art
A distributed feedback type laser diode (hereinafter referred to as a DFB-LD), in which a periodic structure for changing the refractive index (i.e., a diffraction grating) is provided along an optical waveguide path, is indispensable to long-haul high bit-rate optical communication.
As is well known in the art, however, the DFB-LD does not always ensure single-mode oscillation unless the reflection factors of the end facets of a laser cavity are optimally adjusted. (Refer to William Streifer, Robert D. Burnham and Donald R. Scifres, IEEE J. Quantum Electronics, vol. QE-11, pp. 154-161, April, 1975)
In order to reliably ensure single longitudinal-mode oscillation, a phase shift structure is proposed wherein the phase of light is changed inside a laser cavity. (Refer to H. A. Haus and C. V. Shank, "Antisymmetric taper of distributed feedback lasers", IEEE J. Quantum Electronics, vol. QE-12, pp. 532-539, 1975)
A structure in which single-mode oscillation is actually confirmed is a .lambda./4 shift type DFB laser (.lambda.: lasing wavelength). In this type of DFB laser, the phase of a diffraction grating is changed by .pi. (i.e., the phase of an oscillated wave is changed by .pi./2), so as to control the reflection at the end facets. (Refer to Utaka et al., "Electronics Letters", vol. 20, No. 24, 1984, pp. 1008-1010)
An equivalent .lambda./4 phase shift type DFB laser is also proposed, wherein the effective refraction factor of an active medium is changed by varying the width of a waveguide path in the center of a laser cavity. In this type of DFB laser, the phase of light is shifted before and after the optical wave passes through the portion at which the effective phase shifter of the active medium dimension is changed. In this structure the phase of light is shifted distributedly, whereas in the ordinary phase shift type DFB laser the phase is shifted concentratedly by the diffraction grating. (Refer to Soda et al., IEEE J. Quantum Electronics, vol. QE-23, No. 6, pp. 804-814, June, 1987)
As can be seen from the above, the phase shift methods presently known are limited to two methods, one being a method wherein the phase of a diffraction grating is made discontinuous and the other being a method wherein a waveguide path is changed in width and thickness.
A description will now be given of the problems entailed in these two conventional phase shift methods.
First, the .lambda./4 shift type DFB laser, wherein a diffraction grating is shifted, will be described.
In the case where the coupling coefficient .kappa. of the .lambda./4 shift type DFB laser (the coupling coefficient .kappa. represents an optical wave feedback amount) is large, the optical intensity is high in the center of a laser cavity, as indicated by line II in FIG. 6, since optical waves are likely to concentrate at the shifted portion of the diffraction grating. Since this results in axial hole burning, and the single-mode characteristic is adversely affected, the performance of the laser is considerably affected. The problem is attributable to the above-mentioned fact that the phase of light is shifted concentratedly by the diffraction grating. (Refer to Soda et al., "IEEE Journal of Quantum Electronics, vol. QE-23, No. 6, June, 1987)
Conversely, in the case where the coupling coefficient .kappa. is small, the light inside the laser cavity concentrate at the end facets, as indicated by line III in FIG. 6. Since the light intensity distribution in the axial direction of the laser cavity is not uniform in this case as well, the threshold current increases and the suppression ratio of the side mode decreases.
As mentioned above, it is very difficult to adjust the coupling coefficient .kappa. of the .lambda./4 shift type DFB laser.
A variety of methods for shifting a diffraction grating during its manufacturing process are proposed. For example, "Electronics Letters", vol. 20, No. 24, pp 1008-1010, 1984 discloses a method wherein positive resist and negative resist are used, and Research Report of Electronic Information Communication Society, OQE86-150 discloses a method wherein a phase shift film is used. However, none of these known methods ensure a high yield of manufacture.
The methods wherein a diffraction grating is shifted have problems in that a step section is undesirably produced at the shifted portion of the diffraction grating and in that the diffraction grating has different shapes between one and the other sides of the shifted portion.
As described above, many problems remain unsolved with respect to the phase shift structures wherein the diffraction grating is shifted.
A description will now be made of the structure wherein phase shift is equivalently produced by changing the width of a waveguide.
If the phase shift amount in this structure is .lambda./4, the concentration of optical waves in the phase shift region can be significantly prevented in comparison with the above-mentioned case where the phase of the diffraction grating is made discontinuous. This is because the length of the phase shift region is normally several tens of .mu.m.
In addition, the diffraction grating used in this phase shift structure can be easily manufactured since its shape is uniform and continuous. Therefore, it can be said that this phase shift structure is more advantageous than the phase shift structure wherein the diffraction grating is shifted.
However, it is difficult to form even a waveguide having a uniform and constant width since the width must be controlled with high precision. This being so, it is more difficult to form a waveguide having different widths. As a matter of fact, it is extremely difficult to accurately control the width and shape of the phase shift portion of the waveguide. Therefore, it is not easy to accurately adjust the phase shift amount.
Since the equivalent refractive index of the active layer considerably varies at the width-changed portion of the waveguide, the lasing mode and the radiation mode (from the equivalent refractive index-varying portion) interfere with each other. As a result, the far-field pattern of light output is likely to become very irregular, as shown in FIG. 8. The radiation angle of the radiation mode cannot be accurately known beforehand and is therefore difficult to control. For coupling to another optical element, such as an optical fiber, it is desirable that the far-field pattern have no irregularity and narrow expansion angle.
As a single-mode oscillation structure for use in a distributed feedback type laser element (which is a type other than the phase shift type structures described above), a structure wherein the period of the diffraction grating is continuously changed in a laser cavity is proposed. Also, a structure wherein the width of a waveguide is continuously changed to form a tapered waveguide is proposed. These two structures give solution to the problem that the far-field pattern becomes distorted.
With respect to the structure wherein the period of the diffraction grating is continuously changed, the manufacture of the diffraction grating (i.e., a chirped diffraction grating) and the determination of the diffraction grating period suitable for the element become problems in practice. As for the structure wherein the width of the waveguide is continuously changed, the control of the width of the waveguide is not easy, as mentioned above.
As a method wherein phase shift is equivalently produced by changing the structure of a waveguide, Jpn. Pat. Appln. KOKAI Publication No. 61-88584 proposes a method for changing the thickness of the waveguide layer.
According to the method proposed in the Japanese publication, a groove corresponding to a phase shift is formed in a substrate, and the thickness of the subsequently grown waveguide layer is changed. However, the diffraction grating formed on the substrate is inevitably flattened by the etching process presently available. It is therefore not easy to permit the diffraction grating to remain in the phase shift region.
In addition, the portion where the diffraction grating is located and the other flat portion have different crystal orientations at their exposed surfaces. As a result, the waveguide layer subsequently grown has not only different crystalline characteristics but also different thicknesses, depending upon locations. Therefore, there is inevitably a step section in the active layer.
Further, since the control of the thickness of the grown waveguide layer is difficult, it is not possible to obtain a desirable phase shift. For these reasons, the method proposed in the Japanese publication is not practical.
It is thought to form the diffraction grating after the waveguide layer is grown. This method may enable easy control of the phase shift amount, since the layer structure is determined when the diffraction grating is formed.
However, if the diffraction grating is formed after the thickness of the waveguide layer is changed, the coated photoresist is inevitably thicker at the depressed portion of the step section than at the projected portion. In the depressed portion, therefore, the diffraction grating has different shapes or depths, and in the worst case no diffraction grating is formed.
As described above, it is very difficult to control the shape and depth of the diffraction grating not only at the projected portion but also at the depressed portion.
In the meantime, a method for producing a number of phase shifts in a laser cavity is proposed so as to obtain a narrow spectral line width for the purpose of coherent light communication. (Refer to Kimura et al., "Electron Lett.", vol. 23, 1987, pp. 1014)
However, the manufacture of such an element structure is very difficult as long as the conventional phase shift methods are applied.
As has been described, not only the conventional method for producing phase shift at a single point but also the conventional method for equivalently providing a phase shift region in the axial direction of a laser cavity is limited in advantage, controllability and productivity.
In summary, the conventional methods or structures have the following disadvantages:
(1) It is difficult to control the coupling coefficient .kappa. and structural parameters in laser elements of distributed feedback type and distributed Bragg reflector type.
(2) The structure wherein phase shift is equivalently produced inevitably has a stepped section, and this stepped section produces adverse effects. That is, the far-field pattern becomes distorted due to the radiation mode, the phase shift region cannot be formed with high precision, and the stepped section cannot be processed easily.
(3) Since the concentration of light at the phase shift region results in axial hole burning, the performance of the element is adversely affected.